Two types of analysis are useful when examining Igneous Rocks:

1. Modal analysis - requires only a thin section,
2. Normative analysis - requires a chemical analysis.

MODAL ANALYSIS

Produces an accurate representation of the distribution and volume percent of the mineral within a thin section. Three methods of analysis are used:

1. Measure the surface area of mineral grains of the same mineral, relative to the total surface area of the thin section.
2. Measure the intercepts of each mineral along a series of lines.
3. POINT COUNT - Count each mineral occurrence along a series of traverse line across a given thin section. For a statistically valid result > 2000 individual points must be counted.

The number of grains counted, the spacing between points and successive traverse lines is dependant on the mean grain size of the sample.

Advantages

1. One can compare rocks from different areas if you only have a thin section, no chemical analysis is required, using a petrographic microscope.
2. Gives the maximum and minimum grain sizes.

Disadvantages

1. Meaningless if the sample has a preferred orientation of one or more minerals.
2. Porphyritic rocks are difficult to count.
3. Total area of sample must be sufficiently larger than the max. diameter of the smallest grain size.

OVERHEAD - MODAL ANALYSIS GRAPH AND DATA